1. Field of the Invention
The present invention relates to digital systems. More specifically, the present invention relates to digital filters used to control bandwidth in a communication system.
2. Description of the Related Art
Modern communication systems have evolved from using predominantly analog circuitry to predominantly digital circuitry over the past several years. Where there use to be passive and active analog circuits used to oscillate, mix, filter and amplify signals in ‘analog’ domain, there are now digital signal processing circuits that employ specialized microprocessors, DSP's and related circuits to process signals in the digital domain. At the circuit component level, digital circuits are much more complex, employing millions of devices in some circumstances. However, at the device level, digital communications circuits are much simpler, often times using just a handful of devices to accomplish a complex communications task. Portable wireless devices are good examples of this high level of integration. Digital communications circuits also provide the substantial benefit of programmability. A single device can serve many functions over time, allowing the system designer to closely tailor the function of the communications system to the needs of the effort at hand.
One particular functional area that has benefited from the transition to the digital domain is signal filtering. As system designs have become more stringent, with narrowing bandwidths and higher information rates, analog filter topologies had become very complex, expensive, and prone to parts and circuit tolerance limitations. Analog filters become rather poor choices when narrow bands, close channelization, and high information rates force the need for very high order filters to meet system design criteria. Fortunately, the advent of digital signal processing and digital filter theory has alleviated this problem to a great degree. However, digital filters are not without their own limitations, and the state of the art has evolved to the point where even digital filter designs are challenged to meet tight system requirement.
A particular family of filters that is commonly employed in digital communication systems implemented with digital filtering technology is the finite impulse response, or ‘FIR’, filter. These filters are often characterized by linear phase response and constant group delay without feedback. In a typical implementation, a FIR filter response is implemented as a number of taps in a time domain delay line, each tap having an associated coefficient that defines the filter response characteristics. The number of taps indicates the order of the filter, as well as the amount of processor overhead that is required to implement the filter. The implementation of a FIR filter in a digital signal processor is widely understood by those skilled in the art. In fact, commercial software applications exist that allow designers to enter desired filter response parameters and then quickly produce filter tap coefficients that meet the design characteristics. Digital signal processing devices offer low level instructions designed to make filter implementation as efficient as possible.
As is understood by those skilled in the art, digital filters, like any filter, can be represented in the frequency domain or the time domain. In the frequency domain, segments of the filter transfer function are delineated as the pass band, stop band and transition band in a typical high-pass, low-pass, or band-pass filter. The frequency domain can be readily transformed to the time domain. In the time domain, the filter response is represented by an impulse function. A filter designed in the frequency domain with a fixed frequency cutoff has a theoretically infinite time impulse response to fully realize the cutoff frequency. Since time is always constrained, the impulse function must be truncated. However, truncating the time domain necessarily results in a broadening, or splattering, of energy bandwidth in the frequency domain. Where a filter is used to control bandwidth, as in channelizing a communications signal, this splattering of energy can result in undesirable interference, noise, reduced system performance, and violation of FCC regulations. The problem is of particular concern in any system where the communications of information must be started and stop with any regularity. A digital filter requires time to ramp up and produce useful output. Thus, there is a period of time at the beginning and end of each transmission block of information which does not contribute to the communications of useful information through the system. In effect, the data throughput performance of the system is compromised by the filter's limitations.
There are certain techniques available to those skilled in the art for controlling this limitation of digital filter systems. One technique is to further truncate the filter at the beginning and ends of transmission periods. This results in reduced system noise immunity and spectral spreading for those periods, but can be employed to advantage none the less. Another technique is to reduce the ramp-up and ramp-down periods for the filter and control the resultant spectral spreading by truncating and windowing the data for the ramp periods. In effect, the energy is forced to zero at the very beginning and ending moments of a time slot of signal transmission. Even given these techniques, the system designer is forced to exchange spectral efficiency for data bandwidth performance in such systems. Thus, there is a need in the art to improve data throughput, by reducing ramp up and ramp down time performance in digital communication filters while maintaining control of spectral performance.